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// Copyright 2017 The LUCI Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
// Package readonly implements a filter that enforces read-only accesses to
// datastore.
//
// This is useful in hybrid environments where one cluster wants to read from
// a cache-backed datastore, but cannot modify the cache, so reads are safe and
// direct, but writes would create a state where the cached values are invalid.
// This happens when mixing AppEngine datastore/memcache with Cloud Datastore
// readers.
package readonly
import (
"golang.org/x/net/context"
ds "go.chromium.org/gae/service/datastore"
"go.chromium.org/luci/common/errors"
)
// ErrReadOnly is an error returned in response to mutating datastore
// operations.
var ErrReadOnly = errors.New("readonly: datastore is read-only")
// readOnlyDatastore is a datastore.RawInterface implementation that returns
// ErrReadOnly on mutating operations.
type readOnlyDatastore struct {
ds.RawInterface
isRO Predicate
}
type perKeyCB func(idx int, err error) error
type implCB func(mutable []int, cb perKeyCB) error
func (r *readOnlyDatastore) run(keys []*ds.Key, impl implCB, cb perKeyCB) error {
var mutable []int
if r.isRO != nil { // all keys are RO by default
for i, k := range keys {
if !r.isRO(k) {
mutable = append(mutable, i)
}
}
}
if len(mutable) != 0 {
if err := impl(mutable, cb); err != nil {
return err
}
}
cur := 0 // cursor inside 'mutable' array
for idx := 0; idx < len(keys); idx++ {
if cur != len(mutable) && idx == mutable[cur] {
cur++
continue // results for 'idx' was already delivered above
}
if err := cb(idx, ErrReadOnly); err != nil {
return err
}
}
return nil
}
func (r *readOnlyDatastore) AllocateIDs(keys []*ds.Key, cb ds.NewKeyCB) error {
impl := func(mutable []int, cb perKeyCB) error {
mutableKeys := make([]*ds.Key, len(mutable))
for i, idx := range mutable {
mutableKeys[i] = keys[idx]
}
return r.RawInterface.AllocateIDs(mutableKeys, func(idx int, key *ds.Key, err error) error {
return cb(mutable[idx], err)
})
}
return r.run(keys, impl, func(idx int, err error) error {
return cb(idx, keys[idx], err)
})
}
func (r *readOnlyDatastore) DeleteMulti(keys []*ds.Key, cb ds.DeleteMultiCB) error {
impl := func(mutable []int, cb perKeyCB) error {
mutableKeys := make([]*ds.Key, len(mutable))
for i, idx := range mutable {
mutableKeys[i] = keys[idx]
}
return r.RawInterface.DeleteMulti(mutableKeys, func(idx int, err error) error {
return cb(mutable[idx], err)
})
}
return r.run(keys, impl, perKeyCB(cb))
}
func (r *readOnlyDatastore) PutMulti(keys []*ds.Key, vals []ds.PropertyMap, cb ds.NewKeyCB) error {
impl := func(mutable []int, cb perKeyCB) error {
mutableKeys := make([]*ds.Key, len(mutable))
mutableVals := make([]ds.PropertyMap, len(mutable))
for i, idx := range mutable {
mutableKeys[i] = keys[idx]
mutableVals[i] = vals[idx]
}
return r.RawInterface.PutMulti(mutableKeys, mutableVals, func(idx int, key *ds.Key, err error) error {
return cb(mutable[idx], err)
})
}
return r.run(keys, impl, func(idx int, err error) error {
return cb(idx, keys[idx], err)
})
}
// Predicate is a user-supplied function that examines a key and returns true if
// it should be treated as read-only.
type Predicate func(*ds.Key) (isReadOnly bool)
// FilterRDS installs a read-only datastore filter in the context.
//
// This enforces that mutating datastore operations that touch keys for which
// the predicate returns 'true' fail with ErrReadOnly.
//
// If the predicate is nil, all mutating operations are denied.
func FilterRDS(c context.Context, p Predicate) context.Context {
return ds.AddRawFilters(c, func(ic context.Context, inner ds.RawInterface) ds.RawInterface {
return &readOnlyDatastore{inner, p}
})
}